Heat stabilizers epoxides

Mention has already been made of epoxide stabilisers. They are of two classes and are rarely used alone. The first class are the epoxidised oils, which are commonly employed in conjunction with the cadmium-barium systems. The second class are the conventional bis-phenol A epoxide resins (see Chapter 22). Although rarely employed alone, used in conjunction with a trace of zinc octoate (2 parts resin, 0.1 part octoate) compounds may be produced with very good heat stability. 

The methylated maleic acid adduct of phthalic anhydride, known as methyl nadic anhydride VI, is somewhat more useful. Heat distortion temperatures as high as 202°C have been quoted whilst cured systems, with bis-phenol epoxides, have very good heat stability as measured by weight loss over a period of time at elevated temperatures. The other advantage of this hardener is that it is a liquid easily incorporated into the resin. About 80 phr are used but curing cycles are rather long. A typical schedule is 16 hours at 120°C and 1 hour at 180°C. 

The tendency for PVC to degrade by the unzipping type of dehydrochlorination is reduced by the presence of heat stabilizers, such as barium and cadmium salts of high-molecular-weight carboxylic acids and epoxidized unsaturated aliphatic esters. The general reaction for cadmium salts is as follows ... 

Phthalates, 4a in particular, are certainly the most frequently applied plasticizers. Esters of dicarboxylic acids, such as 8 impart very good flexibility at lower temperatures. Aliphatic oligomeric esters are excellent fat-resistant plasticizers. Epoxidized plasticizers provide some co-stabilizing effect to heat stabilizers in PVC and improve oil and water resistance. 

An additive system was developed for poly(vinyl chloride) for medical applications. The additives include primary stabilisers (Ca-Zn stearate and Zn stearate), secondary stabilisers (epoxides) and lubricants (ethylene bisamide and high density polyethylene), to improve melt processing and heat stability. The use of the stabilisers resulted in reduced equipment down-time, increased the level of recycled material which could be incorporated, and enhanced the product characteristics, including colour, clarity, blush, aqueous extractables and particle generation. 5 refs. 

The variety of substances used as additives in polymers is considerable. For example, the fillers may include china clay, various forms of calcium carbonate, talc, silicas (diatomaceous silica), silicates, carbon black, etc. The impact modifiers typically include other polymers. Plasticizers include certain polymers with low (oligomers), dialkyl phthalates, dialkyl sebacates, chlorinated paraffin waxes, liquid paraffinic fractions, oil extracts, etc. Heat stabilizers include heavy metals salts such as basic lead carbonate, basic lead sulfate, dibasic lead phosphite (also acting as a light stabilizer), dibasic lead phthalate, stearates, ricinoleates, palmitates and octanoates of cadmium and barium, epoxide resins and oils, amines, diphenylurea, 2-phenylindole, aminocrotonates. The antioxidants include tris-nonyl phenyl phosphite, 2,6-di-ferf-butyl-p-cresol (BHT), octadecyl-3,5-di-terf-butyl-4-hydroxyhydrocinnamate, etc. The UV stabilizers include modified benzophenones and benzotriazoles. Processing lubricants include calcium stearate, stearic acid, lead stearate, various wax derivatives, glyceryl esters and long-chain acids. Fire retardants include antimony oxide, some pyrophosphates, etc. 

When used in PVC organophosphites act as secondary heat stabilizers. In other resins such as PP, PS, HOPE and ABS plastics they function as antioxidants. p-Diketones are well-known co-stabilizers more particularly for zinc carboxylates. The synergistic action of epoxidized fatty acid esters like ESO or butylepoxy stearate as plasticizer, HCl scavenger and stabilizer for labile chlorine atoms in the catalytic presence of Zn and Cd salts is shown in Fig. 7 (see also 5.2.3)... 

Acid acceptors are used when acids may be involved in the degradation of the polymer. Calcium stearate is often used with polypropylene to neutralize any mineral acids, such as hydrochloric acid, that might result from the reaction of water with catalyst residues. Various mixed metal laurates are used as stabilizers in polyvinyl chloride. Efforts are underway to replace the cadmium in some of these mixtures (see Chap. 4). Epoxidized soybean oil has been used as the acid acceptor, along with dioctyltinbis(thiogly-colate), to improve the heat stability of polyvinyl chloride.27... 

Development of epoxidized soya oil, as an acid-absorbing and low-volatile flexibilizing agent, provided the quantum jump in heat stability. High concentrations of epoxidized soya oil could be used as a replacement for much of the primary plasticizers with attendant improvement in the composition s thermal tolerance. 

In films for food packaging, only FDA-approved plasticizers can be used. Most often, an adipate plasticizer, usually di(2-ethylhexyl) adipate (DEA), combined with epoxidized soybean oil is used. The oil epoxide is frequently termed a secondary plasticizer, because it is not used alone, and has heat-stabilizer properties in addition to plasticization effects. 

Aliphatic esters are diesters of adipic, sebacic or azelaic acids and offer greater resistance to low temperatures than phthalates, but at a higher cost Epoxy ester plasticizers have limited compatibility with PVC so are used at low concentrations. The most widely used epoxy ester plasticizer is epoxidized soya bean oil (ESBO). ESBO resists extraction, does not migrate readily and acts also as a heat stabilizer. Other plasticizers in this group include epoxidized linseed oil and tall oil. 

Epoxidized oils function as heat stabilizers in chlorine-containing polymers by scavenging liberated hydrogen chloride. 

The phosphite esters function in this formulation as primary heat stabilizers by replacing labile chlorides on PVC via the Arbuzov rearrangement mechanism. When phosphites are used at optimized levels in combination with catalytic amounts of zinc and normal levels of epoxidized soybean oil, they provide excellent early color and adequate long-term stability to flexible PVC compounds. Their use also helps in lowering stabilization cost, elimination of toxic metals, elimination of plate out, poor printability, viscosity drift in plastisols, haze, bloom, water blush, and staining. 

Chetn. Descrip. Epoxidized soybean oil CAS 8013-07-8 EINECS/ELINCS 232-391-0 Uses Stabilizer and plasticizer for all PVC formulations, film and sheet for agric., transparent food pkg., cable, gasket, hose, tube, paint stabilizer for org. halogen such as neoprene Features Provides exc. heat and weathering stability rec. for improving heat stability in Ba-Zn or Ca-Zn formulation Regulatory FDA and JHPA compliance... 

Market Volume. The total market for PVC heat stabilizers may be about 100 million pounds in the United States and 1 biUion pounds worldwide, half for organotin and half for metal soap-epoxidized fatty ester-organic phosphite systems. 

Beta diketones. Beta diketones are secondary heat stabilizers used in combination with Ca/Zn and Ba/Zn metallic heat stabilizers to improve initial color. Beta diketones usually require the presence of other secondary heat stabilizers such as epoxidized oils and organophosphites. 

Epoxy ester. Epoxy ester plasticizers have limited compatibility with PVC. Therefore, they are used at low levels. Epoxidized soybean oil (ESO), the most widely used epoxy plasticizer, is also used as a secondary heat stabilizer. As a plasticizer, it provides excellent resistance to extraction by soapy water and low migration into adjoining materials that tend to absorb plasticizers. Other epoxy plasticizers include epoxidized linseed oil and epoxidized tall oils. Tall oils are prepared from tall oil fatty acids and Cs-Cs alcohols. 

Plastics, particularly chlorine-containing polymers, are susceptible to thermal decomposition when exposed to high temperatures or prolonged heating. There are several kinds of stabilizers on the market. The most important contain lead, tin, calcium and zinc or barium and zinc. Epoxidized oils and esters are also used. Diphenylthiourea is used as a heat stabilizer in PVC. 

Other stabilizers include special zincs for plasticized applications such as cable and organics added to calcium/zinc or used for pipe extrusion and co-stabilizers for metal soap and tin, improving long-term heat stability. Kickers (liquid barium/zinc and potassium/zinc) are PVC stabilizers that catalyse the decomposition of a blowing agent to be effective at lower temperatures, and can be used for sponge leather calendering. Epoxidized compounds are effective co-stabilizers in most systems chelators with metal soap improve heat stability. 

---